For clinical patients with suppressed hematopoiesis, introduction of autologous hematopoietic stem cells (HSC) that can be manipulated and expanded in culture would be very beneficial. However, primitive HSC with pluripotent potential and long term repopulating ability are presently difficult to maintain in culture; they tend to mature into lineage- committed progenitors. The long term goal of this proposal is to demonstrate that the recently characterized mammalian protein Cyr61, a recombinant form of which can be purified in large quantities, will enhance expansion of pluripotent human HSC in culture. The short term goal is to investigate whether murine Cyr61 can function as a supplement to a murine HSC culture system. The specific aims and design of the project will be to establish long term murine HSC cultures with or without supplemental Cyr61, and assay the populations of these cultures for growth rate, and relative proportions of immature stem cells and more mature progenitors. Demonstration of the ability of Cyr61 to enhance expansion of HSC in culture would lead to attempts to use these cells to effect in vivo hematopoietic repopulation in Phase II.